The present invention relates to a diesel engine comprising a device for controlling the flow of injected fuel.
In a common-rail diesel engine 10 (FIG. 1), each cylinder 12, 14, 16 and 18 has a combustion chamber 1312, 1314, 1316 or 1318 into which a fuel is injected by means of an injector 2012, 2014, 2016 or 2018 connected to the common rail 22. In the latter, the fuel is maintained at high pressure by a pump connected by a duct 26 to the tank (not shown) of the vehicle, thus permitting injections of fuel into each cylinder at high pressures, generally comprised between 200 and 1600 bars.
The operation of each injector 20i is controlled by a unit 28 which determines the amount of fuel injected by this injector into the chamber 13i. For this, the unit 28 receives information such as the torque C required by the driver of the vehicle or the pressure P of the fuel in the common rail, and accordingly sets an opening or activation period of the injector 20i such that the latter injects into the chamber 13i the amount of fuel necessary to obtain the torque required by the driver.
This activation period is determined according to the amount of fuel that is to be injected into the chambers and according to the fuel pressure in the common rail. The activation period furthermore depends on the characteristics of the injector; this is because it is preset by the designer, for example by experiment. However, it is known that the operation of an injector equipping a vehicle differs from its preset operation. In fact, the preset operation is established by means of an injector taken as a model, making no allowances for the tolerances accepted when the injectors are machined, nor especially of wear, as shown below by FIG. 2 representing the operation of an injector 20i after a certain time of operation, for example the injector 2012, and that of an injector model used in determining the data stored in the unit 28.
In this FIG. 2, the amount of fuel injected in milligrams (axis of ordinates 32) into the combustion chamber of cylinder i by the model injector (curve 34) in relation to the duration (axis of the abscissae 30) of the activation of this injector established in microseconds. Curve 36 corresponds to the injection performed by the injector 2012 used.
Beginning at curve 34, it is found at first that, for a model injector, there is a minimum activation period ΔTMA from the issuance of the injection command (instant T28) before the fuel injection starts in the chamber and, in a second period the amount of fuel injected varies linearly in relation to time, this relationship between the amount of fuel injected and the activating period being referred to hereinafter as the operating gradient of the model injector.
Curve 36 shows that, for the real injector 2012 used, the minimum activation period is longer: ΔTMA and Δt. Moreover, the operating gradient of the injector 2012 is not as great as that of the injector taken as model.
These differences adversely affect the operation of the engine. Indeed, when an injector operates with an offset Δt and/or with a modified operating gradient, the activation periods controlled by unit 28 cause the injection of an amount of fuel into the combustion chambers different from the predetermined optimum amount. For example, considering that an amount Km (FIG. 2) of fuel is to be injected into the cylinder 12, the unit 28 sets a period of activation ΔTK determined from the curve 34. However, the injector 2012 then actually introduces a quantity K12 of fuel determined by the curve 36, which is perceptibly less than the amount expected, on account of differences of operation due to tolerances, to machining and/or to wear on the injectors.
The higher the fuel pressure is, the more these differences are undesirable, for when this pressure increases the difference in the amounts of fuel injected between an injector—for example one that has been operating for some time—and a model injector generally increase.
These differences impair performance (torque and power), increase combustion noise and/or increase pollutant emissions of the engine, particularly the nitrogen oxides.
These problems can affect all the cylinders of an engine or each cylinder separately.